If course, if they already trust each other (including all the used tools), then the whole excercise is moot.
They would need to trust that the bridge is not malicious. And that is a whole other rabbit hole. But I think it is possible for them to attest the firmware/gateware running on it, through some convoluted cryptographic ceremony.
They'd get a live notification of tampering, and could have independent signal blocksers that could physically block the command from actually arriving at the drive (assuming they use a few hundred meters of coiled fiber as a delay line).
The receiver on the drive side could even be a single phtotodiode, which could be made to allow easy verification with, say, an electron microscope if you're really paranoid. There are probably ways to use field-suitable technology if you only need to ensure the photodiode has the same structure as what you expect.
Cryptography won't help you with trusting hardware. Delays and intervention-ability would help, though.
But in any case, this is not really the level of concern here. It's equipment that tampers with the device. The only way to be sure is to roll your own. Which holds for both sides. So the perfect systems needs to be created by two adverse parties, which means it's impossible to do. Qed.
(In the real world with physical proof this is different since tampering is much harder and it's a problem worked on for centuries. It's not bullet proof either but much more mature.)